1. Field of the Invention
The present invention relates to a display panel unit, comprising a set of cells, which is a display device having a memory function, and to a data transmission system for transmitting display signals from a main unit to the display panel unit. More particularly, this invention is concerned with a display for performing gray-scale display according to a subfield method, a display panel unit constituting the display, and a display signal generator that is incorporated in the main unit for supplying display signals to the display panel unit.
2. Description of the Related Art
An alternating current (hereinafter AC) plasma display panel (PDP) is typical of a display device having a memory function. The present invention can apply to displays for performing gray-scale display according to a subfield method and is not limited to displays using an AC plasma display panel. Herein, however, description will proceed by taking a display using an AC plasma display panel for instance.
The AC PDP is designed to sustain discharge by applying a voltage alternately to two sustaining electrodes and to thus glow for display. One discharge is completed in one to several microseconds after application of a pulse. Ions that are a positive charge generated by discharge are accumulated on the surface of an insulating layer over one electrode to which a negative voltage is applied. Likewise, electrons that are a negative charge are accumulated on the surface of an insulating layer over another electrode to which a positive voltage is applied.
After a pulse (writing pulse) of a high voltage (writing voltage) is used to induce discharge and produce a wall charge, and when a pulse (sustaining pulse or sustaining discharge pulse) of a lower voltage (sustaining voltage or sustaining discharge voltage) whose polarity is opposite to that of the previous voltage is applied, the previously-accumulated wall charge is duplicated. Consequently, a voltage to be induced in a discharge space increases and eventually exceeds a threshold of a discharge voltage. Discharge then starts. In short, a cell has a feature that once a wall charge is produced by performing one writing discharge, when a sustaining pulse is applied by alternating the polarity, discharging is sustained. This feature is referred to as a memory effect or memory function. In general, the AC type PDP achieves display by utilizing the memory effect.
A PDP cannot vary the intensity of glowing. Luminance is substantially varied by changing the period of glowing, whereby gray-scale display is achieved. Gray-scale display in the PDP is usually achieved by associating each bit of display data with a period of a subfield and varying the length of a subfield according to the degree of weighting each bit. Taking 256-level gray-scale display for instance, display data is composed of eight bits. One frame is displayed for the period of eight subfields. Each bit data is displayed for an associated subfield. The ratio of lengths of subfields is 1:2:4:8:16:32:64:128. One subfield is divided into a reset period, addressing period, and sustaining discharge period. During the reset period, a whole-screen writing pulse is applied to execute self-erasure discharge. All cells in a panel assume a uniform state devoid of a wall charge. During the addressing period, addressing discharge is executed line-sequentially so that a quantity of wall charge permitting sustaining discharge is stored in cells to be allowed to glow. Thus, cells are tuned on or off according to display data. Thereafter, sustaining discharge is executed and an image for one subfield is displayed. In this "addressing/sustaining discharge-separated writing addressing system," luminance is determined with the length of a sustaining discharge period; that is, the number of sustaining pulses. For brighter display, the sustaining discharge period within a frame must be made longer.
For gray-scale display, one frame must be composed of several subfields associated with different glowing frequencies. The same number of addressing cycles as the number of display lines is needed for each subfield.
As described above, a display device that cannot vary the glowing intensity achieves gray-scale display according to a subfield method. It is therefore required to supply display data to a driver in a panel in a format matched with the subfield method.
In an existing large-screen multicolor display type flat-panel display, a digital RGB interface conformable to the specifications of input timing for a CRT is generally adopted. This is because the main unit of the existing display usually outputs signals adapted to a CRT interface and the signals are intended to be shared with the main unit. A display interface is therefore provided with a CRT controller (CRTC) and video RAM. Display data is developed in the video RAM, and then read and output in conformity with the specifications of input timing for a CRT.
As mentioned above, in the known display, the main unit transmits display data to a display panel unit in a format adapted to the CRT interface. In this case, a frame memory must be installed in the main unit and display panel unit respectively. The frame memory must have a capacity that is large enough to store bits representing gray-scale levels for each display pixel. When the number of pixels is large, an enormous memory area becomes necessary. In particular, for color display, three frame memories must be installed. The necessary memory area gets larger. In addition, when it is attempted to unintermittently display data of frames that are input consecutively, the display panel unit is requested to write data of the next frame while reading out written data from a frame memory. The same number of frame memories as the number of a plurality of frames must therefore be prepared so that the frame memories can be switched in order to perform writing and reading concurrently. Consequently, the necessary memory area further expands.
As mentioned above, the display for performing gray-scale display according to the subfield method has a problem in that the frame memory area is large. This poses a problem of increase in cost.